Hmm, can you reinstall firefox but at the same time keep all your bookmarks and favourites? I have too many it's not worth reinstalling if I lose them...
I just tried Internet Explorer, and YouTube comments worked, so perhaps I can just use IE for YT...
Edit: You know what actually, I remember now a Firefox update a few days ago... I am sure it's this update that causes the problem, it was working fine before...
Maybe I will just wait for next Firefox update or email them...